There are a variety of devices used to treat fractures of bones, such as the femur, humerus, and tibia. For example, fractures of the femur have been successfully treated with an orthopaedic implant, such as an intramedullary nail longitudinally placed within the medullary canal to connect the bone fragments. Such implants typically include a plurality of openings, such as circular holes and elongated slots, which receive fasteners to attach the implant to the cortical surface of the bone. While circular holes prevent the fasteners from translating in the axis of the nail, elongated slots can allow the fasteners to slide, translate, or dynamize, with respect to the axis of the nail. Some holes are threaded to prevent the nail from sliding or translating over or along the axis of the fastener. “Dynamization” refers to a movement of the fastener and the nail relative to each other in a direction generally parallel to the axis of the nail. Such dynamization can promote healing of the bone by putting the fracture site under stress.
Due to the fact that slots in the nail commonly have no threads in order to accommodate dynamization, the nail can also translate along the axis of the fastener in cases where either there are no threaded holes near the slots or the surgeon chooses not to insert fasteners into the threaded holes near the slots. Such translation of the nail along the axis of the fastener can be referred to as the “windshield wiper effect,” which is undesirable and can lead to bone instability. What is needed is an orthopaedic implant system that will prevent, for example, the intramedullary nail from translating in the axis of the fastener while still allowing the fastener to dynamize in or along the axis of the nail. “Trans-fixation” refers to an implant that can dynamize over the fastener relative to one axis while being fixated to the fastener relative another axis.
Prior attempts have tried to use threaded slots with threaded sidewalls that engage the threads of the fastener such that the fastener can be displaced and locked within the slot for intraoperative fracture compression purpose. However, such approaches do not allow the implant, for example the nail, to dynamize once the fastener is locked in the threaded slot. Furthermore, due to the engagement of the threads of the fastener and those of the slot, dynamization of the nail in this situation will rotate the fastener on its axis, resulting in the fastener screwing in and out of the bone. This is undesirable as it causes severe pain in patients. Other disadvantages of prior attempts include that they require the threads of the slot to precisely or substantially conform to the threads of the fastener, thus limiting the types of fasteners that can be inserted into the threaded slot. Additionally, the fastener generally needs to be inserted into the threaded slot at a precise angular orientation in order for the threads of the slot and the threads of the fastener to engage each other properly.